Recently, thin film magnetic heads are being put to practical use, and are known to have properties higher than, by one order of magnitude or more, those of the conventional magnetic heads prepared by using soft ferrite blocks as the starting plates. In order to improve the properties of such thin film magnetic heads, it is also proposed to form a stepped structure in the cross-section of the thin magnetic films thereof.
However, since the thin film heads of the stepped structure are of a low strength with respect to the sliding movement of magnetic recording media, provision has been made of thin film magnetic heads using magnetic substrates having therein a groove filled with a non-magnetic layer, whereby the distance between the opposite poles is magnetically increased, and the substrate is physically (geometrically) of a flush surface. In general, glass is used as the non-magnetic layer.
The currently available substrates used for that purpose are ones of a diameter of 76.0 mm (maximum) .times. a thickness of no more than 4 mm and ones with a square side of no more than 50.8 mm .times. a thickness of 1-2 mm. In the near future, however, use may possibly be made of substrates of larger dimensions. For example, the dimensions of the grooves are of a width of 10-100 microns.times.a depth of 10-50 microns for the consumer or personal purpose, and of a width of 0.1-0.5 mm .times. a depth of 10-100 microns for use in computers.
In order to embed glass into the surface of the magnetic substrates such as Mn-Zn ferrite, Ni-Zn ferrite or the like in the groove form, there is a process wherein plate-like glass is placed in at least one groove formed in the surface of the magnetic substrate, and heated to a temperature at which a glass viscosity reaches 10.sup.4 -10.sup.5 poise, thereby to form a non-magnetic layer. However, such procedures pose problems in that pores easily appear in the formed glass layer, and, when Mn-Zn ferrite is used as the magnetic substrate, an interdiffusion layer of the substrate and glass components is formed in a region where they come into contact with each other.
In the preparation of thin film magnetic heads, a glass layer is formed on a substrate, and, thereafter, several kinds of thin films defining a magnetic circuit are formed on the precision finished glass layer with the use of IC technology. However, if pores are produced in such a glass layer, disconnection or poor insulation of the magnetic circuit, and/or a variation in magnetic properties will be obtained due to the exposed pores. Additionally, the yield of the head product will also be dependent largely upon the number of pores, thus posing a grave problem in view of quality control.